pH, neutralisation and titrations
Acids and alkalis are defined by the ions they release in water:
- Acid → releases H⁺ ions in solution.
- Alkali → soluble base that releases OH⁻ ions in solution.
The pH scale measures how acidic or alkaline a solution is.
The pH scale
- 0–6: acidic (lower pH = stronger acid).
- 7: neutral (pure water).
- 8–14: alkaline (higher pH = stronger alkali).
Universal indicator changes colour through red → orange → yellow (acidic) → green (neutral) → blue → purple (alkaline). A pH probe/meter gives a numerical value — more accurate.
Neutralisation
When acid + alkali react, the H⁺ and OH⁻ combine:
H⁺(aq) + OH⁻(aq) → H₂O(l)
This is the ionic equation for all neutralisations. The other ions (the spectator ions) form the salt:
HCl + NaOH → NaCl + H₂O
Required practical: titration of HCl with NaOH
Aim: find the exact volume of dilute HCl needed to neutralise 25.0 cm³ of NaOH solution of known concentration.
Apparatus: burette, pipette + filler, conical flask, white tile, methyl orange indicator (or phenolphthalein).
Procedure:
- Use a pipette to transfer 25.0 cm³ of NaOH solution into a conical flask.
- Add a few drops of methyl orange indicator (yellow in alkali).
- Fill burette with HCl, take initial reading at the meniscus.
- Run HCl into the flask, swirling, until the indicator just turns red (end-point).
- Record final burette reading. Calculate titre = final − initial.
- Repeat for concordant results (within 0.1 cm³). Take the mean.
The end-point is when neutralisation is complete: equal moles of H⁺ and OH⁻ have reacted.
Indicator choice
- Methyl orange: yellow (alkaline) → red (acidic). Sharp colour change.
- Phenolphthalein: pink (alkaline) → colourless (acidic). Sharp colour change.
- Universal indicator gives a gradual change, so it's not used for titrations.
✦Worked example— Worked example — concentration calculation (HT links to C3.7)
In a titration, 25.0 cm³ of 0.10 mol/dm³ NaOH is exactly neutralised by 20.0 cm³ of HCl. Find concentration of HCl.
- moles NaOH = 0.025 × 0.10 = 0.0025
- mole ratio NaOH : HCl = 1 : 1, so moles HCl = 0.0025
- conc HCl = 0.0025 / 0.020 = 0.125 mol/dm³
⚠Common mistakes
- Using universal indicator — gradual colour change unsuitable for end-point.
- Reading meniscus from wrong angle — read at eye level, bottom of meniscus.
- Not rinsing burette with the acid before filling — dilutes it.
- Forgetting to swirl — incomplete mixing leads to overshooting.
- Forgetting to repeat for concordancy.
Common acids and bases (memorise)
| Acid | Formula | Anion |
|---|---|---|
| Hydrochloric | HCl | Cl⁻ |
| Sulfuric | H₂SO₄ | SO₄²⁻ |
| Nitric | HNO₃ | NO₃⁻ |
| Base | Formula | Cation |
|---|---|---|
| Sodium hydroxide | NaOH | Na⁺ |
| Potassium hydroxide | KOH | K⁺ |
| Ammonia | NH₃ (NH₄OH in soln) | NH₄⁺ |
Links
Builds on C4.4 (acid reactions). Sets up C4.8 (strong vs weak acids HT) and C3.7 (concentration calculations HT).
AI-generated · claude-opus-4-7 · v3-deep-chemistry